Screw tightening structure, screw member, and screw tightening tool

ABSTRACT

A threaded fastener capable of being reduced in size and weight. A pair of side wall surfaces ( 18 ) of each of six radially outwardly projecting, torque transmitting portions ( 16 ) lie on straight lines passing through the center line (O) in a cross section perpendicular to the center line (O) so as to transmit torque through the side wall surface ( 18 ), whereby the direction of a surface orthogonal force N applied to a force-applied point P by a wrenching tool coincides with the direction of an effective wrenching force F. This arrangement reduces an operation force required for rotating the wrenching tool and accordingly the load acting on each of the torque transmitting portions, in comparison with a conventional hexagonal head ( 100 ). That is, it is possible to reduce the diameter of the head portion ( 12 ) of the threaded fastener ( 10 ) and the size and weight of the head portion ( 12 ), while avoiding deformation of the side wall surfaces ( 18 ) through which the torque is transmitted.

TECHNICAL FIELD

[0001] The present invention relates to a threaded-fastener wrenchingstructure, and more particularly to such a threaded-fastener wrenchingstructure which leads to reduction in size and weight of a threadedfastener.

BACKGROUND ART

[0002] Conventionally, a threaded fastener having a hexagonal head isgenerally widely used. In a wrenching operation of such a threadedfastener having the hexagonal head, however, a driving angle α (see FIG.10) is 60°, namely, there is generated a component of force whichcomponent is not directed for wrenching the threaded fastener. It istherefore difficult that the hexagonal head is made compact or reducedin its diameter and axial height. As shown in FIG. 10, the driving angleα, which is 60° in case of the hexagonal head 100, corresponds to anangle between the direction of a surface orthogonal force N and thedirection of an effective wrenching force F, wherein the surfaceorthogonal force N corresponds to a force applied to a force-appliedpoint Q (corresponding to a vertex of the hexagon) while the effectivewrenching force F corresponds to a force effective for wrenching thethreaded fastener. A wrenching torque T is expressed by an expression(1) where “F” and “r” represent the effective wrenching force and aradius of the hexagonal head, respectively. In this instance, as isapparent from an expression (2), the surface orthogonal force N appliedto the force-applied point Q is twice as large as the effectivewrenching force F. Meanwhile, an expansive reaction force E, whosedirection coincides with a radial direction of the hexagonal head, isabout 1.7 times as large as the effective wrenching force F. If theradius r of the hexagonal head requires to be reduced without reductionin the wrenching torque T, the effective wrenching force F has to beincreased inversely with the reduction of the radius r. For increasingthe effective wrenching force F, the surface orthogonal force N and theexpansive reaction force E have to be increased. That is, it will benecessary to increase an operation force for rotating the wrenchingtool, thereby causing a risk of eroding the force-applied point Q, i.e.,a corner portion of the hexagonal head 100.

T=6×F×r  (1)

N=F/cos 60°=2F  (2)

E=F×tan 60°≈1.7F  (3)

[0003] With respect to “Hexagonal head bolt” of JIS B1180 and “Hexagonalhead bolt with washer flange” of JIS B1189, their dimensions s, e, k asindicated in FIGS. 11 and 12 are specifically defined in relation with anominal diameter of thread as shown in FIG. 13. The dimension srepresents a diameter of a circle inscribed in the contour of thehexagonal head. The dimension e represents a diameter of a circlecircumscribed about the hexagonal contour. The dimension k represents aheight of the hexagonal head. A ratio of the circumscribed circlediameter e with respect to the nominal thread diameter d is representedby “e/d”. A ratio of the height k of the hexagonal head with respect tothe nominal thread diameter d is represented by “k/d”. As is apparentfrom FIG. 13, the circumscribed circle diameter e is not smaller than1.55d, and the height k of the hexagonal head is not smaller than 0.6d.

[0004] The present invention was made under the above-describedbackground with object of providing a threaded-fastener wrenchingstructure which makes it possible to reduce the size and weight of athreaded fastener, without reducing the wrenching torque applied to thethreaded fastener.

DISCLOSURE OF INVENTION

[0005] For achieving the object, a first invention is athreaded-fastener wrenching structure comprising: a fitting hole whichis provided in one of a threaded fastener and a wrenching tool forwrenching the threaded fastener; and a fitting protrusion which isprovided in the other of the threaded fastener and the wrenching tooland which is to be brought into fitting engagement with the fittinghole; wherein each of the fitting hole and protrusion has a contourhaving a plurality of torque transmitting portions which areequi-angularly spaced apart from each other about a center line of theeach of the fitting hole and protrusion and which project outwardly in aradial direction of the each of the fitting hole and protrusion, so thata wrenching torque is transmitted through the torque transmittingportions to the threaded fastener, as a result of rotation of thewrenching tool about the center line when the fitting hole andprotrusion are held in fitting engagement with each other, thethreaded-fastener wrenching structure being characterized in that (a)each of the torque transmitting portions has an engaging portion throughwhich the wrenching torque is transmitted to the threaded fastener,wherein a tangent line tangent to the engaging portion substantiallycoincides with a straight line passing through the center line in across section perpendicular to the center line, and that (b) each of thetorque transmitting portions has a radially outer portion which has, ina plan view thereof, a fan shape whose center is the center line,wherein a portion of the contour between each circumferentially adjacentpair of the torque transmitting portions defines a pentagonal groove ora U-shaped groove having a smoothly curved bottom surface.

[0006] A second invention is a threaded-fastener wrenching structurecomprising: a fitting hole which is provided in one of a threadedfastener and a wrenching tool for wrenching the threaded fastener; and afitting protrusion which is provided in the other of the threadedfastener and the wrenching tool and which is to be brought into fittingengagement with the fitting hole, wherein each of the fitting hole andprotrusion has a contour having a plurality of torque transmittingportions which are equi-angularly spaced apart from each other about acenter line of the each of the fitting hole and protrusion and whichproject outwardly in a radial direction of the each of the fitting holeand protrusion, so that a wrenching torque is transmitted through thetorque transmitting portions to the threaded fastener, as a result ofrotation of the wrenching tool about the center line when the fittinghole and protrusion are held in fitting engagement with each other, thethreaded-fastener wrenching structure being characterized in that (a)each of the torque transmitting portions has an engaging portion throughwhich the wrenching torque is transmitted to the threaded fastener,wherein a tangent line tangent to the engaging portion substantiallycoincides with a straight line passing through the center line in across section perpendicular to the center line, that (b) the threadedfastener has an external thread, that the fitting protrusion is providedin the threaded fastener, and that the fitting protrusion has a maximumdiameter Dmax, a minimum diameter Dmin and a height k as measured in adirection parallel with the center line such that the maximum diameterDmax, the minimum diameter Dmin and the height k satisfy the followingexpressions (4), (5), (6) which represent relationships with a majordiameter d of the external thread:

Dmax≦1.5d  (4)

1.1d<Dmin  (5)

0.3d≦k<0.45d  (6)

[0007] A third invention is a threaded-fastener wrenching structurecomprising: a fitting hole which is provided in one of a threadedfastener and a wrenching tool for wrenching the threaded fastener; and afitting protrusion which is provided in the other of the threadedfastener and the wrenching tool and which is to be brought into fittingengagement with the fitting hole, wherein each of the fitting hole andprotrusion has a contour having a plurality of torque transmittingportions which are equi-angularly spaced apart from each other about acenter line of the each of the fitting hole and protrusion and whichproject outwardly in a radial direction of the each of the fitting holeand protrusion, so that a wrenching torque is transmitted through thetorque transmitting portions to the threaded fastener, as a result ofrotation of the wrenching tool about the center line when the fittinghole and protrusion are held in fitting engagement with each other, thethreaded-fastener wrenching structure being characterized in that (a)each of the torque transmitting portions has an engaging portion throughwhich the wrenching torque is transmitted to the threaded fastener,wherein a tangent line tangent to the engaging portion substantiallycoincides with a straight line passing through the center line in across section perpendicular to the center line, that (b) the threadedfastener has an external thread, that the fitting protrusion is providedin the threaded fastener, and that the fitting protrusion has a maximumdiameter Dmax and a minimum diameter Dmin, both of which are smallerthan a major diameter d of the external thread.

[0008] The tangent line tangent to the engaging portion substantiallycoincides with the straight line passing through the center line. Thismeans that the intersection angle between these lines is not larger than±3°. The engaging portion may be provided by at least a portion of eachof the torque transmitting portions. It is ideal that the tangent lineand the straight line passing through the center line completelycoincide with each other. It is preferable that the intersection anglebetween these lines is not larger than ±1°.

[0009] A seventh invention is, in the threaded-fastener wrenchingstructure defined in any one of the first through third inventions,characterized in that the engaging portion is a flat surface having apredetermined dimension as measured in a direction of the tangent line.

[0010] An eighth invention relates to a threaded fastener, and ischaracterized by being provided with the fitting hole or the fittingprotrusion defined in the first invention.

[0011] A ninth invention relates to a threaded fastener, and ischaracterized by having an external thread and being provided with thefitting protrusion defined in the second or third invention.

[0012] A tenth invention relates to a wrenching tool, and ischaracterized by being provided with the fitting hole or the fittingprotrusion defined in the first invention.

[0013] In the threaded-fastener wrenching structure, each of the torquetransmitting portions has the engaging portion through which thewrenching torque is transmitted to the threaded fastener, wherein thetangent line tangent to the engaging portion substantially coincideswith the straight line passing through the center line in the crosssection perpendicular to the center line. In this arrangement, theabove-described driving angle α is substantially 0°, namely, thedirections of the surface orthogonal force N and the effective wrenchingforce F substantially coincide with each other, while the expansivereaction force E is substantially 0°. Therefore, a required amount ofthe surface orthogonal force N is smaller than that in the case of thehexagonal head shown in FIG. 10, thereby making it possible to reducethe operation force required for rotating the wrenching tool andaccordingly the load acting on each of the torque transmitting portions.That is, it is possible to reduce the diameter of the fitting hole orprotrusion without reducing the wrenching torque, while avoidingdeformation of the engaging portion. Thus, the threaded fastener or thewrenching tool can be made with reduction in its size and weight.

[0014] In the seventh invention, there is provided the flat surfacehaving a predetermined dimension as measured in the direction of thetangent line, namely, there is provided the engaging portion which isflat and extends in a straight line passing substantially through thecenter line. Therefore, the effective force F is distributed over theflat surface, leading to reduction in a load per unit area of thesurface. Thus, the threaded fastener or the wrenching tool can be madewith further reduction in its size and weight.

[0015] In the second invention, the fitting protrusion provided in thethreaded fastener having the external thread has the maximum diameterDmax, the minimum diameter Dmin and the height k which satisfy theabove-described expressions (4), (5), (6). Therefore, the size of thefitting protrusion in relation with the major diameter d of the externalthread is reduced in comparison with a conventional hexagonal head,while deformation or fracture of each torque transmitting portion isavoided. Thus, the threaded fastener is made with reduction in its sizeand weight.

[0016] In the threaded fastener of the eighth invention and thewrenching tool of the tenth invention, it is possible to obtainsubstantially the same effect as in the above-described first invention.In the threaded fastener of the ninth invention, it is possible toobtain substantially the same effect as in the second or thirdinvention.

[0017] In the threaded-fastener wrenching structure, it would be commonthat the threaded fastener such as a hexagonal head bolt is providedwith the fitting protrusion while the wrenching tool is provided withthe fitting hole. However, the structure may be adapted such that thethreaded fastener is provided with the fitting hole (recess) while thewrenching tool is provided with the fitting protrusion (blade). Althougheach of the fitting hole and protrusion may be formed to besubstantially parallel with the axial direction, each of the fittinghole and protrusion may be adapted to include an inclined portion or achamfered portion which are included with respect to the center line, inthe interest of facilitating its manufacture with forging process andfacilitating its fitting engagement in the wrenching operation.

[0018] In general, the number of the outwardly projecting, torquetransmitting portions in each of the fitting hole and protrusion isappropriately five or six. However, the number may be smaller than fiveor larger than six.

[0019] The engaging portion, whose tangent line substantially coincideswith the straight line passing through the center line in the crosssection perpendicular to the center line, is preferably provided by theflat surface having a predetermined dimension as measured in thedirection of the tangent line, as in the second invention. It ispreferable that the engaging portion of each of the torque transmittingportions is provided by a pair of side wall surfaces each of which has across sectional shape represented by a straight line extending towardthe center line away from the radially outer portion. However, each ofthe side wall surfaces does not have to be flat, but may be curved tohave an arcuate shape in its cross section such that a line tangent to aportion of the side wall surface substantially coincides with thestraight line passing through the center line.

[0020] In the third invention, the minimum diameter Dmin and even themaximum diameter Dmax are smaller than the major diameter d. In such acase, the height k may be adapted, as needed, to be not smaller than0.45d such that the fitting protrusion is given a predetermined degreeof fracture strength.

[0021] In the second invention, the maximum diameter Dmax is adapted tobe not larger than 1.5d and the height k is adapted to be smaller than0.45d, for the purpose of making these dimensions smaller than those ofa conventional hexagonal head bolt. The height k is adapted to be notsmaller 0.3d, for the purpose of making the fracture strength of thehead portion, i.e., the fitting protrusion larger than the fracturestrength of the externally threaded portion when the threaded fasteneris wrenched.

[0022] Further, preferably, the fitting protrusion is provided in thethreaded fastener, wherein it is desirable that a central angle θ1 ofeach of the torque transmitting portions and a central angle θ2 of aportion of the contour between each adjacent pair of the torquetransmitting portions satisfy the expression (7) described below. As amaterial of the wrenching tool, in “SocketWrench-Socket” of JIS B4636,for example, it is prescribed that the tool should be formed of SCM435of JIS G4105 or its equivalent. The SCM435 belongs to strength grade10.9, and has tensile strength of about 1040 N/mm². Meanwhile, it iscommon that the threaded fastener is formed of a material belonging tostrength grade 8.8 and having tensile strength of 800 N/mm². Where thethreaded fastener is constructed such that the expression (7) issatisfied, the torque transmitting portions of the threaded fastener areprevented from being fractured. It is noted that the above-describedterm “strength grade”, which is used in the following description, isdefined in “Mechanical properties of steel bolts and screws” of JISB1051.

1.3≦θ1/θ2≦1.4  (7)

BRIEF DESCRIPTION OF DRAWINGS

[0023]FIG. 1 is a set of views showing a threaded fastener which is oneembodiment of the present invention, wherein (a) is a plan view as seenfrom a head portion of the threaded fastener, while (b) is a crosssectional view taken along line B-B of (a).

[0024]FIG. 2 is a set of views showing a wrenching tool used forwrenching the threaded fastener of FIG. 1, wherein (a) is a bottom planview as seen from a socket portion of the wrenching tool, while (b) is afront view as seen in a direction perpendicular to the center line S.

[0025]FIG. 3 is a table indicating an example of ratio (τ_(B)/Rm) of ashearing strength τ_(B) to a tensile strength Rm, in each strength gradeof material for the threaded fastener.

[0026]FIG. 4 is a table indicating a minimum height kmin, in eachstrength grade of material, which was obtained by using the ratio(τ_(B)/Rm) of FIG. 3, and also a ratio (kmin/d) in the strength grade12.9, by way of example.

[0027]FIG. 5 is a set of views explaining another embodiment of theinvention, and corresponding to FIG. 1.

[0028]FIG. 6 is a set of views explaining still another embodiment ofthe invention, and corresponding to FIG. 1.

[0029]FIG. 7 is a set of views explaining another embodiment of theinvention, and corresponding to FIG. 1.

[0030]FIG. 8 is a set of views explaining still another embodiment ofthe invention, and corresponding to FIG. 1.

[0031]FIG. 9 is a set of views explaining still another embodiment ofthe invention, wherein (a) is a plan view as seen from its head portion,while (b) is a front view as seen in a direction perpendicular to itscenter line.

[0032]FIG. 10 is a view showing a hexagonal head of a conventionalhexagonal head bolt.

[0033]FIG. 11 is a set of views showing dimensions s, e, k, d in“Hexagonal head bolt” of JIS B1180.

[0034]FIG. 12 is a set of view showing dimensions s, e, k, d in“Hexagonal head bolt with flange washer” of JIS B1189.

[0035]FIG. 13 is a table indicating examples of the dimensions s, e, kshown in FIGS. 11 and 12, and also “e/d” and “k/d” obtained from valuesof the dimensions s, e, k.

BEST MODE FOR CARRYING OUT THE INVENTION

[0036] There will be described in detail embodiments of the presentinvention, with reference to the drawings.

[0037]FIG. 1 is a set of views showing a threaded fastener 10 which isone embodiment of the present invention, wherein (a) is a plan view asseen from a head portion 12 of the threaded fastener 10, while (b) is across sectional view taken along line B-B of (a). The threaded fastener10 has an external thread 14 which is integrally formed such that theexternal thread 14 is adjacent to the head portion 12 and is concentricwith a center line O. FIG. 2 shows a wrenching tool 30 having a socketportion 32 which is to be brought into fitting engagement with the headportion 12. The threaded fastener 10 is rotated about the center line Oby the wrenching tool 30, so as to be tightened or loosened. The headportion 12 corresponds to a fitting protrusion, and has a contour havingsix torque transmitting portions 16 which projects radially outwardlyand which are equi-angularly spaced apart from each other at an angularinterval of 60° about the center line O. As shown in the plan view ofFIG. 1(a), each of the torque transmitting portions 16 has an outer wallsurface which has, in its cross section, an arcuate shape whose centercoincides with the center line O. Each of the torque transmittingportions 16 further has a pair of side wall surfaces 18 corresponding toan engaging portion. Each of the side wall surfaces 18 is, in its crosssection, represented by a straight line extending from the outer wallsurface toward the center line O. An intersection angle defined by thisstraight line and a straight line passing through the center line O issubstantially 0°. In the present embodiment, this intersection angle isnot larger than ±1°. The outer wall surface of each transmitting portion16 is provided by a part-cylindrical surface which is parallel with thecenter line O. The side wall surfaces 18 of each transmitting portion 16are provided by flat surfaces which are parallel with the center line O.The side wall surfaces 18 of each adjacent pair of the torquetransmitting portions 16 are connected to each other via a pair ofinclined flat surfaces 20 which are inwardly inclined symmetrically witheach other, so that a portion of the contour between each adjacent pairof the torque transmitting portions 16 defines a groove 22 which has apentagonal shape in its cross section. It is noted that the shaperepresented by the plan view of FIG. 1(a) corresponds to the crosssectional shape in a plane perpendicular to the center line 0.

[0038] The head portion 12 has a maximum diameter Dmax, a minimumdiameter Dmin and a height k such that the maximum diameter Dmax, theminimum diameter Dmin and the height k satisfy the above-describedexpressions (4), (5), (6) which represent their relationships with amajor diameter d of the external thread 14. Each of the torquetransmitting portions 16 subtends a central angle θ1, while the portionof the contour between each adjacent pair of the torque transmittingportions 16 subtends a central angle θ2. The central angles θ1, θ2 asmeasured about the center line O satisfy the above-described expression(7). In FIG. 1, Dmax=1.5d, Dmin=1.1d, k=0.35d, θ1/θ2≈1.4. It is notedthat two-dot chain lines in FIG. 1 represents a contour of the hexagonalhead 100 of the hexagonal head bolt defined by JIS B1180, for comparisonwith the contour of the head portion 12 of the threaded fastener 10.

[0039] On the other hand, the socket portion 32 of the wrenching tool 30is provided by a cylindrical member having a fitting hole 34 whosebottom is defined by a bottom wall of the cylindrical member. FIG. 2(a)is a bottom plan view as seen from the socket portion 32, while FIG.2(b) is a front view as seen in the direction perpendicular to a centerline S. The fitting hole 34 has a contour having six torque transmittingportions 36 which project radially outwardly and which areequi-angularly spaced apart from each other at an angular interval of60° about the center line S. In the bottom plan view of FIG. 2(a), eachof the torque transmitting portions 36 has a fan shape, and has theengaging portion in the form of a pair of side wall surfaces 38 each ofwhich is, in its cross section, represented by a straight line extendingin a direction away from the radially outer portion toward the centerline S. The fitting hole 34 has dimensions each of which is adapted tobe slightly larger than a corresponding one of dimensions of the headportion 12, so that the fitting hole 34 can be fitted on the headportion 12. In a wrenching operation in which the threaded fastener 10is wrenched by the wrenching tool 30, the socket portion 32 isconcentrically fitted on the head portion 12 such that the torquetransmitting portions 16 of the head portion 12 are received in therespective torque transmitting portions 36 of the fitting hole 34. Thesocket portion 32 of the wrenching tool 30 is then rotated about thecenter line S, so that the torque is transmitted to the threadedfastener 10 through the side wall surfaces 38, 18.

[0040] In a threaded-fastener wrenching structure constituted by thethreaded fastener 10 and the wrenching tool 30 which are constructed asdescribed above, each of the six torque transmitting portions 16, 36 hasthe side wall surfaces 18, 38 through which the wrenching torque istransmitted to the threaded fastener 10. Since each of the side wallsurfaces 18, 38 lies on a straight line passing through the center lineO, S in the cross section perpendicular to the center lines O, S, thedirection of a surface orthogonal force N applied by the wrenching tool30 to a force-applied point Q coincides with a circumferential directionabout the center line O, namely, the direction of an effective wrenchingforce F which is effective for wrenching the fastener 10, while anexpansive component force E becomes substantially zero. Therefore, arequired amount of the surface orthogonal force N becomes smaller thanthat in the case of the conventional hexagonal head 100, thereby makingit possible to reduce the operation force required for rotating thewrenching tool 30 and accordingly reduce the load acting on each of thetorque transmitting portions 16, 36. That is, it is possible to reducethe diameter of the head portion 12 of the fastener 10 and accordinglyreduce the size and weight of the head portion 12 of the fastener 10,while assuring a predetermined amount of the effective wrenching torqueand avoiding deformation of the side wall surfaces 18, 38.

[0041] Particularly, in the present embodiment in which the flat sidewall surfaces 18, 38 each provided to lie on the straight line passingthrough the center lines O, S, the effective wrenching force F isdistributed over the flat side wall surface, leading to reduction in aload acting on an unit area of the surface. Thus, the head portion 12 ofthe threaded fastener 10 can be made with further reduction in its sizeand weight.

[0042] That is, in the present embodiment in which the head portion 12of the threaded fastener 10 has the maximum diameter Dmax of 1.5d andthe height k of 0.35d, the size of the head portion 12 is considerablysmaller than the conventional hexagonal head 100 as is apparent fromFIG. 1, whereby the fastener 10 can be made with reduction in its sizeand weight.

[0043] In the study of the height k of the head portion 12, the maximumtensile strength Fmax of the external thread 14 and the maximum tensilestrength Fmax of the head portion 12 are respectively expressed by thefollowing expressions (8), (9) in which tensile strength Rm of thematerial of the threaded fastener, effective cross sectional area A(s)of the external thread 14, shearing strength 1B of the material of thethreaded fastener and shearing area Ask (=k×π×d) of the head portion 12are used. Since the maximum tensile strength Fmax of the head portion 12requires to be larger than the maximum tensile strength Fmax of theexternal thread 14, namely, since Fmax (head portion)>Fmax (thread), thefollowing expression (10) is obtained. The minimum height Kmin isexpressed by the following expression (11).

Fmax(thread)=Rm×A(s)  (8)

Fmax(head portion)=τ_(B) ×Ask  (9)

τ_(B) ×k×π×d>Rm×A(s)  (10) $\begin{matrix}\begin{matrix}{{k\quad \min} = {\left( {{Rm} \times {A(s)}} \right)/\left( {\tau_{B} \times \pi \times d} \right)}} \\{= {{A(s)}/\left\lbrack {\left( {\tau_{B}/{Rm}} \right) \times \pi \times d} \right\rbrack}}\end{matrix} & (11)\end{matrix}$

[0044] On the other hand, the ratio (τ_(B)/Rm) of the shearing strengthτ_(B) to the tensile strength Rm of the material of the fastener isdetermined in each strength grade, as shown in the table of FIG. 3,according to “Systematic calculation of high duty bolted joints” definedin VDI 2230 of German Association of Engineers. The table of FIG. 4shows the minimum height kmin obtained from the above-describedexpression (11), for each strength grade in the threaded fasteners 10having different diameters d. The rightmost column of the table of FIG.4 indicates the ratio (kmin/d) in the strength grade 12.9. Therefore,the height k of 0.35d as in the present embodiment provides a sufficientdegree of shearing strength even in case of the strength grade 12.9. Incase of the strength grade 8.8 or 10.9, namely, in alleviated condition,a sufficient degree of shearing strength is obtained even if the heightk is about 0.3d.

[0045] In the present embodiment in which the angle θ1 of each torquetransmission portion 16 and the angle θ2 of each groove 22 satisfy therelationship expressed by the above-described expression (7), eachtorque transmitting portion 16 of the threaded fastener 10 isadvantageously prevented from being fractured. Where the material of thethreaded fastener 10 belongs to the strength grade 8.8 while thematerial of the wrenching tool 30 is, as defined in“SocketWrench-Socket” of JIS B4636, SCM435 of JIS G4105 which belongs tothe strength grade 10.9, namely, where the tensile strength of thematerial of the threaded fastener 10 is about 800 N/mm² while thetensile strength of the material of the wrenching tool 30 is about 1040N/mm², the fracture strength of the threaded fastener 10 is larger thanthat of the wrenching tool 30 if the expression (7) is satisfied.

[0046] Next, there will be explained other embodiments of the presentinvention. It is noted that the same reference numerals as used in theabove-described embodiment will be used to identify the substantiallysimilar portions, which will not be explained in detail.

[0047]FIG. 5 shows a threaded fastener 40 in which five torquetransmitting portions 16 are equi-angularly spaced apart from each otherabout the center line O, wherein the dimensions Dmax, Dmin, k, θ1, θ2are set to satisfy the above-described expressions (4), (5), (6) and(7).

[0048]FIG. 6 shows a threaded fastener 42 in which a recess 44 is formedin the head portion 12 of the threaded fastener 10. The recess 44 hassix grooves corresponding to the torque transmitting portions 16. Thisthreaded fastener 42 can be wrenched by using two kinds of wrenchingtools. It is noted that the recess 44 is not essential for the presentinvention and does not have to have an engaging portion whichsubstantially coincides with a straight line passing through the centerline O.

[0049]FIGS. 7 and 8 show threaded fasteners 46, 48 in each of which thepentagonal grooves 22 are replaced by U-shaped grooves 52 each having asubstantially semi-circular-shaped bottom surface 50 that is curvedsmoothly from the flat side wall surfaces 18. Each of the grooves 52 haswidthwise opposite edges that are rounded. Each of the threadedfasteners 46, 48 provides the same effect as the threaded fasteners 10,40.

[0050]FIG. 9 shows a studbolt 60 to which the principle of the presentinvention is applied. The studbolt 60 includes a threaded portion 68which has a pair of external threads 64, 66, and a head portion 62 whichserves as the fitting protrusion and which is coaxial with the threadedportion 68. The head portion 62 has, in its plan view, the sameconfiguration as the above-described head portion 12. Thus, like thehead portion 12, the head portion 62 has a contour having the six torquetransmitting portions 16 each having the pair of side wall surfaces 18.However, either of the diameters Dmax, Dmin is smaller than the majordiameter d of the external threads 64, 66. In this embodiment, too,since the torque is transmitted through the flat side wall surfaces 18lying on the straight lines passing through the center line O in theplan view, namely, since the direction of the force (=surface orthogonalforce N) applied to each side wall surface 18 is the same as thedirection of the effective wrenching force F, each side wall surface 18is being prevented from being deformed, even if a required amount of thewrenching force F is increased due to the reduction of the size of thehead portion 62. It is noted that FIG. 9(a) is the plan view as seenfrom the head portion 62 while FIG. 9(b) is the front view as seen inthe direction perpendicular to the center line.

[0051] The embodiments of the present invention have been explained indetail with reference to the drawings. However, each of the embodimentsis merely an embodied form, and the present invention can be embodiedwith various modifications and improvements on the basis of knowledge ofthose skilled in the art.

1. (Deleted)
 2. A threaded-fastener wrenching structure comprising: afitting hole which is provided in one of a threaded fastener and awrenching tool for wrenching said threaded fastener; and a fittingprotrusion which is provided in the other of said threaded fastener andsaid wrenching tool and which is to be brought into fitting engagementwith said fitting hole; wherein each of said fitting hole and protrusionhas a contour having a plurality of torque transmitting portions whichare equi-angularly spaced apart from each other about a center line ofsaid each of said fitting hole and protrusion and which projectoutwardly in a radial direction of said each of said fitting hole andprotrusion, so that a wrenching torque is transmitted through saidtorque transmitting portions to said threaded fastener, as a result ofrotation of said wrenching tool about said center line when said fittinghole and protrusion are held in fitting engagement with each other, saidthreaded-fastener wrenching structure being characterized in that eachof said torque transmitting portions has an engaging portion throughwhich the wrenching torque is transmitted to said threaded fastener,wherein a tangent line tangent to said engaging portion substantiallycoincides with a straight line passing through said center line in across section perpendicular to said center line, that said threadedfastener has an external thread, that said fitting protrusion isprovided in said threaded fastener, and that said fitting protrusion hasa maximum diameter Dmax, a minimum diameter Dmin and a height k asmeasured in a direction parallel with said center line such that saidmaximum diameter Dmax, said minimum diameter Dmin and said height ksatisfy the following expressions (4), (5), (6) which representrelationships with a major diameter d of said external thread:Dmax≦1.5d  (4) 1.1d<Dmin  (5) 0.3d≦k<0.45d  (6).
 3. (Deleted) 4.(Amended) A threaded-fastener wrenching structure according to claim 2,wherein an intersection angle between said tangent line and saidstraight line passing through said center line is not larger than ±3°.5. (Amended) A threaded-fastener wrenching structure according to claim2, wherein an intersection angle between said tangent line and saidstraight line passing through said center line is not larger than ±1°.6. (Amended) A threaded-fastener wrenching structure according to claim2, wherein said plurality of torque transmitting portions consist offive or six torque transmitting portions.
 7. (Amended) Athreaded-fastener wrenching structure according to claim 2,characterized in that said engaging portion is a flat surface having apredetermined dimension as measured in a direction of said tangent line.8. (Deleted)
 9. (Amended) A threaded fastener, characterized by havingan external thread, and being provided with the fitting protrusiondefined in claim
 2. 10. (Deleted)
 11. A threaded-fastener wrenchingstructure according to claim 2, characterized in that said fittingprotrusion is provided in said threaded fastener, wherein a centralangle θ1 of each of said torque transmitting portions and a centralangle θ2 of a portion of said contour between each adjacent pair of saidtorque transmitting portions satisfy the following expression (7):1.3≦θ1/θ2≦1.4  (7).